Scientists use the analogy of a skater. When he pulls in his arms, he spins faster.

That's because pulling in his arms changes the distribution of the skater's mass and therefore the speed of his rotation.

Richard Gross, a geophysicist at NASA's Jet Propulsion Laboratory in Pasadena, California, used a computer model to determine how the magnitude 8.8 quake that struck Chile on February 27 may have affected Earth.

He determined that the quake should have moved Earth's figure axis about 3 inches (8 centimeters). The figure axis is one around which Earth's mass is balanced. That shift in axis is what may have shortened days.

Such changes aren't unheard of.

The magnitude 9.1 earthquake in 2004 that generated a killer tsunami in the Indian Ocean shortened the length of days by 6.8 microseconds.

On the other hand, the length of a day also can increase. For example, if the Three Gorges reservoir in China were filled, it would hold 10 trillion gallons (40 cubic kilometers) of water. The shift of mass would lengthen days by 0.06 microsecond, scientists said.